> You could use a trick. Sequences returned from a sequence 
> expression are always flattened and concatenated. So:
> 
> for $city in ('Hamburg', 'Coblenz', 'Aachen') return (if 
> (contains('Hamburger', $city)) then true() else ())
> 
> ...returns true(), because the empty sequences are simply 
> discarded. 

Not a good idea. If the input was "von Hamburg nach Aachen" the result would
be (true(), true()), and applying boolean() to that result gives an error.

And you don't need a trick here anyway, because 

some $city in ('Hamburg', 'Coblenz', 'Aachen') satisfies (contains($input,
$city))

solves the problem perfectly well.

Or, if you prefer brevity, you can write

exists(('Hamburg', 'Coblenz', 'Aachen')[contains($input, .)])


Michael Kay
http://www.saxonica.com/